The present invention relates to a semiconductor, a solar cell employing the semiconductor, a method of manufacturing the solar cell, and a solar cell unit employing the solar cell.
For the approximately the past ten years, grate attention has been paid to solar cells (solar batteries) employing silicon as a power source which is harmless to the environment. As for these solar cells employing silicon, a monocrystalline silicon type solar cell is known, which is used in artificial satellites or the like. In addition, a practical application of a solar cell is also known employing polycrystalline silicon (single crystal silicon) and a solar cell employing amorphous silicon. These solar cells have already been practically used in industrial and household applications.
However, since these solar cells employing silicon are manufactured through a vacuum process, such as a CVD (chemical vapor deposition) process or the like, manufacturing cost is high. Further, since a great deal of quantity of heat and a great deal of electricity are used in the manufacturing process, the balance between the energy required for manufacturing the solar cell and the energy generated by the solar cell is very poor. Thus, these solar cells are not yet established as an energy-saving power source.
On the other hand, a new type of solar cell, which is referred to as xe2x80x9cwet solar cellxe2x80x9d or xe2x80x9cfourth-generation photocellxe2x80x9d, was proposed in 1991 by Grxc3xa4tzel et al. As shown in FIG. 9, this wet solar cell includes one electrode 901 formed of titania (titanium dioxide), which is a semiconductor, and another electrode 902 formed of platinum, ITO or the like, and these electrodes are held in an electrolyte solution 903, such as an iodine solution.
The reaction principle of this wet solar cell is as follows. When receiving rays such as solar rays, the titania (TiO2), which is a semiconductor, receives electrons to deliver them to the electrodes, and holes (h+), which are left in the titania electrode, oxidize iodine ions to convert Ixe2x88x92 into Ixe2x88x923. Then, the iodine ions, which have been oxidized, receive the electrons again to be reduced at the counter electrode. Thus, the iodine ions are cyclically moved between both of the electrodes, thereby realizing the battery.
In this wet solar cell employing such an electrode formed of titania, however, only the ultraviolet rays in the solar rays are efficiently utilized. Therefore, in order to increase sensitivity of the solar cell so as to be able to absorb light up to the visible ray region, the titania is mixed with organic dye or the like. For this reason, such a wet solar cell is called as a dye-sensitized solar cell. Since this type of wet cell can be manufactured from inexpensive materials and does not need a large scale equipment, such as an equipment for the vacuum process and the like for its manufacturing, it is greatly expected that this wet solar cell will be a low cost solar cell.
However, since this dye-sensitized solar cell is a wet cell employing an electrolyte, such as an iodine solution or the like, it is necessary to seal its solar cell containing the iodine solution as the electrolyte with a sealing material. Due to this structure, the dye-sensitized solar cell is subject to many problems in that, for example, leakage of electrolyte solution is liable to occur when the sealing is broken and the like.
Therefore, the dye-sensitized solar cell cannot have a practical life as a solar cell.
Further, current and voltage of practically required levels cannot be secured by simply employing a flat-shaped titanium electrode because of its small absorption area of solar rays.
In view of the above problems, the present invention is directed to a solar cell employing a titanium dioxide semiconductor, which includes a pair of electrodes; and a titanium dioxide semiconductor which is held between the electrodes, the surface and inside of the titanium dioxide semiconductor being formed with pores, and the titanium dioxide semiconductor being arranged so as to form a rectification barrier with respect to at least one of the electrodes.
This makes it possible to provide a solar cell which can secure current and voltage of practically required levels, that is, a solar cell which is excellent in the photoelectric conversion efficiency.
In the present invention, it is preferred that the rectification barrier is formed by contacting the titanium dioxide semiconductor with at least one of the electrodes, and the rectification barrier has a diode characteristic. According to this structure, it is possible to enhance the efficiency of power generation of the solar cell.
Further, it is preferred that the rectification barrier is the shottky barrier being formed by contacting the titanium dioxide semiconductor with at least one of said electrodes. According to this structure, it is also possible to enhance the efficiency of power generation of the solar cell.
Alternatively, it is also preferred that the rectification barrier is the PN junction being formed by contacting the titanium dioxide semiconductor with at least one of said electrodes. This also enables enhancement of the efficiency of power generation of the solar cell.
In this invention, it is also preferred that the electrode, with which said titanium dioxide semiconductor forms the rectification barrier, is formed in such a way as to penetrate into the surface of the titanium dioxide semiconductor and the inside thereof. This makes it possible to further increase the area (surface area) where the rectification barrier is formed, thereby further enhancing the efficiency of power generation of the solar cell.
Further, in the present invention, it is also preferred that the titanium dioxide semiconductor has a porosity of 5 to 90%. This increases the contacting area between the titanium dioxide semiconductor and light (that is, the irradiated area by light), thereby enhancing the efficiency of power generation of the solar cell.
Preferably, the titanium dioxide semiconductor has a porosity of 15 to 50%. This further increases the contacting area between the titanium dioxide semiconductor and light (that is, the irradiated area by light), thereby further enhancing the efficiency of power generation of the solar cell.
More preferably, said titanium dioxide semiconductor has a porosity of 20 to 40%. This further more increases the contacting area between the titanium dioxide semiconductor and light (that is, the irradiated area by light), thereby further more enhancing the efficiency of power generation of the solar cell.
In the present invention, it is also preferred that the titanium dioxide semiconductor is porous and has the fractal structure. This also enhances the efficiency of power generation of the solar cell.
Further, in the present invention, it is preferred that the electrode with which said titanium dioxide semiconductor form the rectification barrier is formed from a transparent electrode made of ITO or the like, or a metallic electrode made of a metal selected from the group consisting of Al, Ni, Cr, Pt, Ag, Au, Cu, Mo, Ti, and Ta, or a metal compound containing therein any one or more of these metals. This also enhances the efficiency of power generation of the solar cell.
Furthermore, it is also preferred that the electrode with which said titanium dioxide semiconductor forms the rectification barrier includes a solid iodide. This also enhances the efficiency of power generation of the solar cell.
Moreover, it is preferred that said titanium dioxide semiconductor forms the rectification barrier includes CuI (copper iodide). This enhances the efficiency of power generation of the solar cell.
Moreover, it is also preferred that said titanium dioxide semiconductor forms the rectification barrier includes AgI (silver iodide). This also enhances the efficiency of power generation of the solar cell.
Further, in the present invention, it is also preferred that the electrodes are formed by vacuum evaporation. This makes it possible to reliably contact the titanium dioxide semiconductor with the electrode so that the efficiency of power generation of the solar cell is further enhanced.
Alternatively, it is also possible to form the electrode using use spattering method. This also makes it possible to reliably contact the titanium dioxide semiconductor with the electrode so that the efficiency of power generation of the solar cell is further enhanced.
Further, it is also possible to form the electrode using use printing method. This also makes it possible to reliably contact the titanium dioxide semiconductor with the electrode so that the efficiency of power generation of the solar cell is further enhanced.
Furthermore, in the present invention, it is also preferred that the titanium dioxide semiconductor is subjected to visual rays absorbable processing for making it possible to absorb visible rays. This makes it possible for the titanium dioxide semiconductor to utilize light in the visual ray region, thereby enabling to enhancement of the efficiency of power generation of the solar cell.
In this case, it is preferred that organic dye is adsorbed to said titanium dioxide semiconductor. This is a preferable means as the visual rays absorbable processing, which enables enhancement of the efficiency of power generation of the solar cell.
In this case, it is preferred that inorganic dye is adsorbed to said titanium dioxide semiconductor. This is also preferable means as the visual rays absorbable processing, which also enables enhancement of the efficiency of power generation of the solar cell.
Alternatively, it is also possible that the inorganic dye being adsorbed to said titanium dioxide semiconductor includes inorganic carbon. This is also a preferable method of performing the visual rays absorbable processing, thereby enabling enhancement of the efficiency of power generation of the solar cell.
In this case, it is also preferred that the inorganic dye being adsorbed to said titanium dioxide semiconductor includes an inorganic matter obtained by dying carbon. This is also a preferable method of performing the visual rays absorbable processing, thereby enabling enhancement of the efficiency of power generation of the solar cell.
In the present invention, it is also preferred that the titanium dioxide semiconductor has oxygen defects. This makes it possible for the titanium dioxide semiconductor to utilize light in the visual ray region, thereby enabling enhancement of the efficiency of power generation of the solar cell.
Alternatively, it is also preferred that the titanium dioxide semiconductor includes impurities such as Cr and/or V. This makes it possible to prevent the crystalline structure of the titanium dioxide semiconductor from being changed.
Another aspect of the present invention is directed to a solar cell unit employing a titanium dioxide semiconductor, which includes a solar cell which includes a pair of electrodes, and a titanium dioxide semiconductor which is held between the electrodes, the titanium dioxide semiconductor being formed with pores, and first and second substrates which holds the solar cell therebetween. According to this structure, it is possible to provide a solar cell unit which can utilize current and voltage of practically required levels, that is a solar cell unit which is excellent in power generation efficiency (photoelectric transfer efficiency).
In this invention, it is preferred that a solar cell unit has first and second substrates arranged so that solar rays enter from the side of one of the substrates, in which the other substrate arranged at the opposite side is coated with a reflection film or has a reflection film thereon. This makes it possible to effectively prevent the passing of light, so that utilizing efficiency of light is further enhanced. With this result, it is possible to enhance the efficiency of power generation of the solar cell.
In this solar cell unit, it is preferred that the first and second substrates are arranged so that solar rays enter from the side of one of the substrates, in which the other substrate arranged at the opposite side is coated with a reflection film or has a reflection film thereon. This arrangement makes it possible to prevent or suppress passing of the light effectively so that the solar cell unit can utilize the light more efficiently. With this result, it is possible to enhance the efficiency of power generation of the solar cell.
Preferably, in this solar unit, the space between the first substrate and the second substrate is filled with an inert gas such as argon gas. This makes it possible to extend the durability of the solar cell unit.
Further, it is also preferred that at least one of the first and second substrates, arranged at the side from which solar rays enter, is formed into a transparent substrate or a translucent substrate formed of glass, plastic or synthetic resin. This arrangement makes it possible to reliably transmit the light to the light receiving surface of the solar cell cell.
Furthermore, in this solar cell unit, it is also preferred that at least one of the first and second electrodes, arranged at the side from which solar rays enter, has a top surface and a bottom surface, and an anti-reflection film is coated or placed on the tip surface or bottom surface. This arrangement makes it possible to reliably transmit the light to the light receiving surface of the solar cell cell and enables enhancement of the efficiency of power generation of the solar cell. With this result, it is possible to enhance the efficiency of power generation of the solar cell.
Moreover, in this solar cell unit, it is also preferred that at least one of the first and second electrodes, arranged at the side from which solar rays enter, has a top surface, and a light catalyst made of titanium dioxide (TiO2) is coated on or placed on the top surface. According to this structure, it possible to degrade impurity substances (such as deoxidized carbon, and organic matters) by the light catalyst, even in the case where the solar cell unit is used out-doors, it is possible to prevent the surfaces of the first sand or second electrode from being contaminated in a preferred manner.